2391 questions

[brucelee]

Hi to all,
I am taking a 2391 course at minute written exam in june. I have a few questions iI am struggling with if anyone would be kind enough to share there knowledge i would appreciate it
1) when testing selv whats the insulation resistance value before further investigation cant find i gn3 tutor first said 5.0Mohm then 2.0Mohm so confused
2) when asked about first four tests for a new lighting circuit is it
. continuity of protective conductors inc main and supplementary bonding
. Insulation resistance
. polarity live test
.earth fault loop impedence
or would you include dead test polarity by continuity methods
3) when circuits dont meet max Zs two methods of overcoming this would you install rcd and supplementary bonding how would i answer this question or would you say shorten cable length ,increase csa
4) when carrying out continuity of ring final circuit conductors whats the reason you would select this instrument is it because it is capable of measuring the difference e.g.between a 1mm cable and a 10 mm
5) when asked to explain a test on a radial socket outlet circuit do i have to include testing Line-Neutral as well as Line-cpc so as to prove polarity?
6) three outcomes to be recorded on a schedule of test results after conducting ring test
i have more but i leave it at that for now
Think i am getting things mixed up because of all revising any help would be extremely helpful cheers

 

[brucelee]

Ok, decided to investigate further & refresh my memory, in Guidance note 3 (mine is a 16th edition copy) pages 40-41, under the heading Protection by seperation of circuits (SELV) sub heading Testing of SELV circuits it states "Minimum IR as stated in table 2.2, however in practice readings of less than 5 megohms require further investigation"

Let me know if this is different in a 17th update GN3 and I will consider investing in a new copy.

cheers mate i think that is what tutor was getting at
in my 17th edition GN3 it does not state that anywhere all it says is for further investigation when doing ir tests its 2.0M and for selv and pelv it does not say anything
so still confused yours is to 16th regs mine is to 17th and tutor has just mixed me up he now saying there is no specified value i just hope this question doesnt come up 0.5M for selv and if in an area which also contains circuits at low voltage then its 1.0M it doesnt say nothing about further investigation on a selv circuit

 

[Notsosmart]

Thanks to all who are contributing in helping me with the answers to these questions,
I am doing the written exam in june and have been revising for last 16 weeks or more, reading everything i can, doing sample exam papers,course notes.
praying and all the rest.
but some of the info i have is contradicting each other.
for example
1). course notes say routine checks have to be carried out by a elecrically skilled person
GN3 say it does not but they should be able top safely use the installation and be able to identify defects

2). what is the legal status of the inspector and tester
Is it duty holder or competent person

Bruce to help you come to the correct decision:
Duty holder:
Someone who has duties under CDM 2007 including: client, co-ordinator, designer, principal contractor, contractor, etc.
(Generally someone who is responsible for the installations upkeep)

Competent Person:
The person is regarded as competent if they have 'sufficient training and experience or knowledge and other qualities to properly assist the employer to meet his safety obligations.'

3). reasons for selecting instruments for the range of tests. i.e.
low resistance ohm meter
insulation resistance ohm meter
earth fault loop impedence tester
pfc tester etc
4). typical readings when carrying out the tests above

Both of the above should speak for themselves, and you do know the answer's.

5) test procedures how will i know when to include safe isolation and proving dead with a approved voltage indicator

You should be checking safe isloation where necessary before working on any system.

please can anybody clue me up on the above cheers

I would ask your tutor to keep giving you past paper's and to maybe even give you a mock exam with an open marking forum so any contradictions can be ironed out.

 

[brucelee]

I would ask your tutor to keep giving you past paper's and to maybe even give you a mock exam with an open marking forum so any contradictions can be ironed out.

so off what you are saying is legal status of the inspector and tester is a duty holder

 

[Guest123]

so off what you are saying is legal status of the inspector and tester is a duty holder

No....he must be competant.

 

[brucelee]

No....he must be competant.

thanks lenny mate,
i was not sure getting mixed up with all reading and writing i am doing, i have exam in june and have found a few questions i am struggling with i first thought it was competent person then duty holder
thanks mate for keeping it to the point
one more question if you would be so kind
if i am asked for first four tests on a new lighting circuit is it
continuity of protective conductors
insulation resistance
polarity by continuity methods
earth fault loop impedence
this is a question that is confusing me or do i include polarity again and not include zs
if you would be so kind to clear this up do you class polarity as two seperate tests or one

 

[Risteard]

He/she is a competent person but I'm sure title in law is duty holder.

As for your other question, dead and live polarity are completely different tests.

 

[brucelee]

He/she is a competent person but I'm sure title in law is duty holder.

As for your other question, dead and live polarity are completely different tests.

much confusion lenny says competent person
thanks mate
i know they are different tests but if asked the question on first four tests on a new lighting circuit how do you answer whats the correct sequence please

 

[Risteard]

much confusion lenny says competent person
thanks mate
i know they are different tests but if asked the question on first four tests on a new lighting circuit how do you answer whats the correct sequence please

I would put:

Continuity of protective conductors
Insulation resistance
Polarity (dead)
Polarity (live)


Of course if using test method 1 for continuity of cpcs you already have confirmed polarity but I'd list them as above anyway.

 

[brucelee]

I would put:

Continuity of protective conductors
Insulation resistance
Polarity (dead)
Polarity (live)


Of course if using test method 1 for continuity of cpcs you already have confirmed polarity but I'd list them as above anyway.

thanks mate i appreciate your help and everyone else just trying to get everything right in me head before exam
main area i am struggling with is explaining test procedures an what to include ie proving dead etc

 

[Guest123]

The examiner will be looking for "competant" in the exam.


A principal duty holder is the owner/director of a company who has ultimate responsibilty for everything. An employee for instance who carries out insp & test must be competant but he wont be the principal duty holder will he.

 

[brucelee]

The examiner will be looking for "competant" in the exam.


A principal duty holder is the owner/director of a company who has ultimate responsibilty for everything. An employee for instance who carries out insp & test must be competant but he wont be the principal duty holder will he.

thanks mate,
for clearing that up i just trying to sort answers in my head before exam, i appreciate everyones help

 

[brucelee]

hi guys thanks again for posts
well i asked my tutor again about the further investigation on selv he said its not stated in the 17th GN3 so it wont be asked
as for the question on legal status of a inspector and tester about to conduct a test must be a competent person
Scaddons book of inspecting and testing says duty holder and so does Electricity at work regulations say duty holder ????its no wonder so many fail this exam when you get contradictory information like this anyway i have a few more questions please
1). When asked to explain the return path under fault conditions on a TT system external to the installation is it
General mass of earth, supply earth electrode
2). reasons for selecting different instruments for the varios tests
a. low resistance ohmeter
b. insulation resistance ohmeter
c. earth fault loop impedence tester
d. pfc tester or prospective fault current function of a earth fault loop impedence tester (this is what our tutor said to call it ?)
e. rcd tester
3). if i am asked to explain how to carry out a test for continuity of protective conductors on a socket outlet
do i have to include testin between line and neutral (R1+RN) or not ?
4). for calculating the max permitted earth fault loop impedence
for rcds is it in a TN system Zs multiplied by I delta n is less than or equal to 50v
for rcds " " " " TT " Ra " " " "" """""""""""""""""""" 50v
for overcurrent protective devices is it for TN system Zs multiplied by Ia is less than or equal to 50v
" " " " " " " "" " " " " " " " "" " " " " " " " " " TT system Ra multiplied by Ia is less than or equal to 50v
5). When testing a earth electrode what is the distance you move the Potential electrode in either direction from its original position

thanks again for all your help

 

[JUD]

hi guys thanks again for posts
well i asked my tutor again about the further investigation on selv he said its not stated in the 17th GN3 so it wont be asked
as for the question on legal status of a inspector and tester about to conduct a test must be a competent person
Scaddons book of inspecting and testing says duty holder and so does Electricity at work regulations say duty holder ????its no wonder so many fail this exam when you get contradictory information like this anyway i have a few more questions please
1). When asked to explain the return path under fault conditions on a TT system external to the installation is it
General mass of earth, supply earth electrode

Return path = Circuit Protective Conductor -> Main Earthing Terminal -> Earthing Conductor -> Installation Earth Electrode -> General mass of earth -> Source Earth Electrode -> Transformer -> Line conductor

2). reasons for selecting different instruments for the varios tests
a. low resistance ohmeter
b. insulation resistance ohmeter
c. earth fault loop impedence tester
d. pfc tester or prospective fault current function of a earth fault loop impedence tester (this is what our tutor said to call it ?)
e. rcd tester

???????????

3). if i am asked to explain how to carry out a test for continuity of protective conductors on a socket outlet
do i have to include testin between line and neutral (R1+RN) or not ?

No. Connecting Line & Neutral has nothing to do with the continuity of a cpc.

4). for calculating the max permitted earth fault loop impedence
for rcds is it in a TN system Zs multiplied by I delta n is less than or equal to 50v
for rcds " " " " TT " Ra " " " "" """""""""""""""""""" 50v
for overcurrent protective devices is it for TN system Zs multiplied by Ia is less than or equal to 50v
" " " " " " " "" " " " " " " " "" " " " " " " " " " TT system Ra multiplied by Ia is less than or equal to 50v

Max permitted Zs = Nominal Voltage / Fault current needed to operate the protective device in a given time.

e.g. For "instantaneous" operation of a 32A Type B MCB you need a fault current of 160A (page 249 BS 7671). Therefore 230 / 160 = 1.44ohms (page 49 BS 7671). As a rule of thumb the tabulated values in BS 7671 must be multiplied by 0.8 and reading obtained during testing should not exceed this corrected value (1.44 * 0.8 = 1.15). This is because the resistance of a conductor at it's operating temperature will be higher.

For RCDs the max Zs = 50V(touch voltage) / Rating of RCD (I^n).

e.g. 50V / 0.03(30mA) = 1666.66ohms

5). When testing a earth electrode what is the distance you move the Potential electrode in either direction from its original position
6m (Page 47 GN3)


thanks again for all your help

Hope that helps

Not sure what you mean by question 2. The reason you select an instrument for a test is because that's the instrument you use for that test.

 

[brucelee]

Hope that helps

Not sure what you mean by question 2. The reason you select an instrument for a test is because that's the instrument you use for that test.

cheers mate these are a few questions off some sample papers
the first question was the fault path external to the installation

the second about reasons for selecting instruments ie continuity of protectine conductors is expecting low values of resistance and a low resistance ohmeter is capable of this

also when
Max permitted Zs = Nominal Voltage / Fault current needed to operate the protective device in a given time.

e.g. For "instantaneous" operation of a 32A Type B MCB you need a fault current of 160A (page 249 BS 7671). Therefore 230 / 160 = 1.44ohms (page 49 BS 7671). As a rule of thumb the tabulated values in BS 7671 must be multiplied by 0.8 and reading obtained during testing should not exceed this corrected value (1.44 * 0.8 = 1.15). This is because the resistance of a conductor at it's operating temperature will be higher.

For RCDs the max Zs = 50V(touch voltage) / Rating of RCD (I^n).

e.g. 50V / 0.03(30mA) = 1666.66ohms

if its for a fuse or cb not bsen 60898 ie a bs 1361 under 32A is it the 0.4s column i look under in the time current charicteristic tables ie for a bs1361 30A is it 230v/ 200A = 1.15ohms for a TN system
and for a TT system the 0.2s 230v/ 240A = 0.96ohms
if its over 32A then i would use the 5sec column is that right
and if i want to find the Fault curent its nominal voltage / zs

 

[brucelee]

hi all thanks for all your help
1). what are the tests for selv
is it between line and neutral at 250v and lives and protective conductor of circuits in same location
2). what are the tests for selv and lv
is it between lives of selv connected together and lives of lv connected together at 500v, 1Mohm and
between lives of selv and the protective conductor of the lv circuit and any exposed conductive parts in same area/enclosure at 500v 1Mohm
3). What are the scales of test instruments and expected readings
ie low resistance ohmmeter
insulation resistance ohmmeter
earth fault loop impedence tester etc
thanks for everyones help